Migrate carrierwave to activestorage

[kkoehn]

We should consider the following points:

• Migration of native_files to ActiveStorage
• Removal of native_file column
• I removed CodeOcean::File#hash_content since I don't think it was used. If it is indeed unused, the column could be removed as well

lays groundwork for #2702

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.18%. Comparing base (6096098) to head (0855103).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #3099 +/- ##
=======================================
Coverage 70.18% 70.18% =======================================
Files 213 212 -1 Lines 6819 6799 -20 =======================================
- Hits 4786 4772 -14 + Misses 2033 2027 -6 

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